The present invention relates to a power assisted vehicle steering system of the kind in which power assistance to vehicle steering is provided by a fluid pressure operated ram having opposed chambers, fluid flow to which from a pressure source (usually a constant volume output pump) is controlled by a steering valve responsive to a steering input. Fluid flow is controlled to the opposed chambers of the ram to provide a fluid pressure differential therebetween and actuation of the ram to assist in a steering manoeuvre which is intended by the steering input. It is desirable that power assistance which is provided for a steering manoeuvre varies in accordance with changes in vehicle speed whereby as vehicle speed increases the power assistance which is available decreases and vice versa so that, for example, at low vehicle speed maximum power assistance is available to facilitate parking and similar manoeuvres.
A prior proposal for a steering system of the kind mentioned where power assistance is responsive to vehicle speed is disclosed in our Patent Specification G.B. 2,215,684 in which a proportion of fluid flow from a constant volume output pump to the steering valve may be diverted by way of a by-pass to a fluid reservoir. The by-pass includes a valve that is adjustable in response to variations in vehicle speed so that at high vehicle speeds maximum fluid flow is permitted through the by-pass and therefore minimum or zero fluid pressure and power assistance will be provided by the ram while at low vehicle speed a minimum or zero fluid flow is permitted through the by-pass so that the full output and pressure of the pump becomes available to actuate the ram and provide maximum power assistance. It has also been proposed that the by-pass can alternatively be provided to permit fluid flow communication between the opposed chambers of the ram and such communication is controlled by a valve in the by-pass. Again this latter valve is responsive to variations in vehicle speed whereby at high vehicle speeds the by-pass valve permits maximum fluid flow communication between the opposed chambers and thereby minimum or zero power assistance is available while at low vehicle speed the by-pass valve closes or permits minimum fluid flow communication between the opposed chambers for maximum power assistance to be provided by the ram. The present invention is particularly concerned with steering systems of this latter type and has as an object to provide means whereby at relatively high vehicle speeds (where small or no power assistance would normally be available) increased or maximum power assistance can be made available during steering manoeuvres which may be regarded as exceptional at such speeds, for example as may be experienced during the application of a rapid and large steering input torque in steering to evade a hazardous condition.